The movement of certain electrically-operated valves, such as certain fuel injectors, comprises an armature assembly that consists of a valve, in the form of a hardened stainless steel needle, that is joined to a ferromagnetic armature. The needle must be relatively harder than the armature to provide durability for withstanding the repeated impacting of the former's rounded tip with a valve seat proximate a nozzle end of the fuel injector. The armature is relatively softer because it must provide the requisite ferromagnetic properties that allow it to be efficiently magnetically attracted to a solenoid when the solenoid is energized to cause the armature assembly to lift the needle off the valve seat against an opposite spring bias that urges the armature assembly toward a seated condition on the seat.
A known embodiment of armature is a machined part having a larger diameter, thimble-shaped portion that is necked down to a smaller diameter, tubular-shaped portion extending coaxially from the base of the thimble-shaped portion. Joining of the needle to the armature is performed by first inserting the end of the needle opposite the needle tip into the smaller diameter portion of the armature and then crimping them together. Crimping can be performed without interference from the thimble-shaped portion of the armature by making the smaller diameter portion sufficiently long. The sidewall of the thimble-shaped portion has a fairly close fit to an internal bore that forms a portion of an internal fuel passage through the fuel injector, and it may in fact be used for guiding the armature assembly motion. One or more fuel holes are drilled through the end wall of the thimble-shaped portion just outside of the smaller diameter neck portion to provide for fuel to pass on its way through the fuel passage to the nozzle end where it is injected into the engine. It is also typical for one or more annular-shaped needle guides, or washers, to engage the needle for guiding the armature assembly motion, and they too have one or more fuel holes allowing fuel to pass through.
While prevailing manufacturing practices include thorough cleaning of machined parts like the armature, it has been discovered that minute metal fragments, burrs, etc. resulting from machining operations may remain attached during part cleaning only to separate at a later time during use of the fuel injector and then become a potential source of contamination. While any such debris that is internally generated after fabrication of a fuel injector may be promptly flushed from the valve without ill effect, entrapment of a debris fragment between the needle tip and the valve seat may occasion minute fuel leakage from the nozzle when the valve is closed, and given the tendency of regulatory authorities to impose increasingly stringent standards on tailpipe emissions of motor vehicles, even minute leakage can be a significant contributor to undesired emission products. Even if a valve is provided with some internal means of containing separated particles, such as a filter or trap, the inclusion of such a means does add to the unit cost, and at that may be insufficient to guarantee that every mass-produced fuel injector will not be susceptible to a degradation in performance due to this sort of internally originating debris.
Another factor affecting fuel injector performance is the phenomenon known as armature bounce. The occurrence of such bounce due to the needle impacting the seat at closing causes the injector to be open slightly longer than intended, and this too may be a significant contributor to tailpipe emissions when exhaust emission regulations are stringent. Various forms of armature bounce dampers have been proposed, but they too add to the unit cost of a fuel injector. Since bounce is a function of armature mass, reduction in armature mass will contribute to minimizing or eliminating undesired armature bounce.
The present invention relates to a new and improved form of armature that can provide for the elimination of machining operations on it and for reduction in its mass, while still providing both for it to serve as part of the armature assembly guidance scheme and for the needle to be joined to it by a convenient staking operation. Generally speaking, the invention contemplates the armature being fabricated by a powdered-metal process or a metal-injection molding process with a unique integral slot structure that provides both the fuel holes that allow fuel to pass through and also tool access for the staking of the needle to it. Details will be seen in the ensuing description and claims that are accompanied by drawings illustrating a presently preferred embodiment of the invention according to the best mode presently contemplated in carrying out the invention.